Thin film transistors (TFTs) are widely used as the pixel portions of displays, such as liquid crystal displays and organic EL displays, and the switching devices of drive circuit portions. Currently, aiming at higher TFT performance, lower temperature fabrication process, and lower cost, channel layer materials are searched for and examined among broad types of materials. Such channel layer materials include, for example, amorphous silicon (a-Si), polycrystalline silicon (poly-Si), microcrystalline silicon (μc-Si), and organic semiconductors.
In recent years, amorphous oxide semiconductors have been studied as promising channel layer materials. For example, methods for fabricating TFTs using an amorphous In—Zn—O (described as a-IZO) thin film and an amorphous In—Ga—Zn—O (described as a-IGZO) thin film for the channel layer are disclosed in Barquinha et al., J. Non-Cryst. Sol., 352, 1756 (2006), and Yabuta et al., Appl. Phys. Lett., 89, 112123 (2006). TFTs using a-IZO and a-IGZO for the channel layer can be operated at high speed with field-effect mobility ten times or more the field-effect mobility of TFTs using a-Si for the channel layer, though the TFTs using a-IZO and a-IGZO for the channel layer are fabricated at lower temperature.